Now linked to cancer, the toxin was waste from a plastic-making process. Chemical companies often mix such leftovers to create other products to avoid the cost of disposal, says one long-time chemical engineer.

The fumigant manufacturers, Dow and Shell Oil Co., discovered decades ago that 1,2,3-trichloropropane, or TCP, was not effective against worms called nematodes, according to documents in lawsuits filed by a dozen Valley cities against the companies. But they apparently left it in a fumigant anyway.

"TCP was a hazardous waste, not a pesticide," said lawyer Todd Robins, who represents several Valley cities and water agencies. "It did nothing for farmers, but Shell and Dow knowingly used their fumigants as a way to dispose of it."

A Dow representative disputes the lawyer's statement, saying TCP never was intentionally put into the fumigant, called Telone. Nor did the company http://www.blogger.com/img/blank.gifintentionally allow TCP to remain in the fumigant, he said.